This invention relates in general to welding of metals which require protection from ambient gases during welding and, more particularly, to a backside shield adapted to maintain an inert gas over the welding area during and immediately after welding.
During fusion welding of many metals, such as aluminum, aluminum-lithium, titanium, and others, gaseous elements (e.g., oxygen and nitrogen) in the ambient air will react with the molten or hot metal to form oxides and other compounds that disadvantageously affect the physical properties of the weld. It is necessary to exclude these gaseous elements from the weld area until the weld has cooled to the point where contamination can no longer occur.
In the past, this problem was overcome in welding processes such as tungsten arc welding, plasma arc welding and the like, by covering the weld area on both the front and back sides of the weld with an inert atmosphere to exclude ambient air. This could typically achieved in two ways. First, the welding head and the parts being welded can be contained in an enclosure that is filled with an inert gas during welding. While effective when welding small parts, this method is not feasible for welding very large structures, such as large aluminum-lithium space vehicle propellant tanks. In a second method for excluding gases from the weld area, the front side of the weld is shielded with the assistance of a trailing shield device and the backside of the weld is shield with a purge box mounted over the back of the weld joint.
Trailing shields are often used to cover and back side of a weld area when a highly contaminant sensitive material is being welded. These trailing shields are essentially small boxes positioned over the weld area just behind the weld torch to shield the solidifying weld puddle. Shielding gas, such as helium, argon or a mixture thereof is routed into a small box like device and diffused out onto the solidifying weld and part surface.
Backside shields or purge boxes may be used to cover the back side of the weld joint during welding. Stationary purge boxes are typically used with small structures. With large structures, the purge boxes are typically mounted so as to travel with the weld torch.
There are, however, a number of problems with these conventional box-like purge devices. Often it is necessary to inspect the weld bead as it forms and control the path of the weld torch from the back side of the weld. Automated systems, using laser sensors or television cameras, are unable to perform these tasks because the box-like device is closed to proper viewing. In addition the box design requires equipment for monitoring shield gas pressure and oxygen content inside the purge device chamber, which complicates the welding operation and increases the probability of weld defects.
Traveling shield boxes are used in many case, mounted on a carriage connected to the weld head moving mechanism so that both travel along opposite sides in alignment with the weld site as the weld site moves along the assembly being welded. In some cases, however, the weld fixtures necessary for use with certain weld setumps do not provide space for such moving shield boxes.
Thus, there is a continuing need for improved devices for shielding the backside of welds against contamination from ambient air while permitting inspection of the weld as it is formed and can be used with weld fixtures that severely restrict access to the back side of the weld site and do not provide space for automatically moving a shield along the back side of the weld.